The work function of an n-type Fin Field Effect Transistor (FinFET) having a tungsten gate is mainly determined by the n-type work function adjustment layer. However, a functional layer is generally disposed between the n-type work function adjustment layer and the gate, and the functional layer typically uses a p-type work function adjustment layer such as TiN. Thus, the presence of the functional layer may adversely affect the work function of the n-type Fin Field Effect Transistor. In this case, it is desirable that the thickness of the functional layer is as small as possible.
Further, a residue (e.g., F) left in the gate after forming the gate may diffuse into the functional layer and into the n-type work function adjustment layer, which may also adversely affect the work function of the n-type Fin Field Effect Transistor. In this case, it is desirable that the thickness of the functional layer is as large as possible.
Therefore, the thickness of the functional layer is of particular importance for an n-type Fin Field Effect Transistor.